1. Field of the Invention
The present invention relates to an electromagnetic power generating type rotation sensor for detecting the rotational speed of a rotating member such as the wheel of an automobile.
2. Description of the Prior Art
In general several types of sensors are known, such as an electromagnetic power generating type rotation sensor, a magnetic rotation sensor employing a magnetic resistor element or a Hall element, and a photoelectric rotation sensor. All of these sensors types can be used for detecting the rotational speed of a rotating member such as the wheel of an automobile. The electromagnetic power generating type rotation sensor comprises a magnet, a magnetic pole piece and a voltage generating coil integrally contained in a case of metal or resin. The amount of magnetic flux in the magnetic pole piece is changed by the irregular surface of a magnetic gear serving as a magnetic rotator in proximity to the magnetic pole piece for generating an AC voltage across a coil. The AC voltage thus generated is substantially proportionate to the number of revolutions of the magnetic gear while the frequency thereof is correctly proportionate to the number of revolutions of the magnetic gear, whereby the rotational speed can be detected by the generated AC voltage or the frequency thereof.
FIG. 3 is a sectional view showing a conventional electromagnetic power generating type rotation sensor 21 including a magnet 22, a magnetic pole piece 23, a coil 24, a bobbin 25, a housing 26, an output lead wire 27, a bracket 28 of resin for mounting the sensor, a filler member 28a also made of resin and a magnetic gear 29. The magnetic gear 29 is mounted on a drive shaft of an automobile or the like so that the magnetic gear 29 will rotate with the wheel of the automobile.
Magnetic flux of the magnet 22 passes through the magnetic pole piece 23, and the amount of such magnetic flux is changed depending on the closeness of the irregular surface, i.e., projections and recesses of the outer peripheral part 29 of the magnetic rotator. Depending on the rate of change in the amount of magnetic flux as a function or time, an output voltage is generated across the coil 24, so that the rotational speed of the magnetic rotator which closely related to the rotational speed of the wheel, can be detected from the variation in the frequency of the output voltage.
Detection of the rotational speed of the wheel is important for brake control of the wheel such as an antilock control, and hence it is necessary to detect the rotational speed of the wheel in a reliable under various conditions from low-speed to high-speed conditions. The electromagnetic power generating type rotation sensor is employed for an automobile or the like since the same can be manufactured at a relatively low cost and is resistant against temperature change.
Such an electromagnetic power generating type rotation sensor employed for measuring the speed of an autmobile must have a excellent corrosion resistance since the same is mounted on a wheel part where it is exposed to a bad environment. Thus, the magnetic pole piece must be made of an anticorrosive material. Further, such a sensor is adapted to defect the rotational speed of the wheel from the change in the amount of magnetic flux in the magnetic pole piece as hereinabove described, and hence the material for the magnetic pole piece must have an excellent magnetic property, i.e., a good saturation magnetic flux density. Thus, it is preferable that the material for the magnetic pole piece has an excellent magnetic property and an excellent corrosion resistance.
However, a material having a high saturation magnetic flux density is generally inferior in corrosion resistance against rusting or the like, whereby conventional magnetic pole pieces have not sufficiently satisfied both of the aforementioned conditions.